1. Field of the Invention
The present invention relates to methods for the purification of metallurgical grade silicon to produce solar grade silicon and to apparatus for use in such methods.
2. Description of Related Art
Silicon that is used in the manufacture of solar cells must have a purity that is commonly referred to as solar grade silicon, typically having no more than about 5 ppm of impurities. Solar grade silicon is significantly more pure than what is commonly referred to as metallurgical grade silicon (which typically has 10,000 ppm of impurities), but does not need to be as pure as what is commonly referred to as electronic grade silicon (which typically has less than 1 ppb of impurities, which is used to make semiconductor devices.
U.S. Pat. No. 5,182,091 discloses purifying silicon by directing a plasma jet toward the surface of molten silicon. The patent discloses that inert gas (such as argon or helium) as the plasma gas may be mixed with 0.1-10% steam and/or less than 1 gram of silica powder per liter of the inert gas. The plasma is generated by using a mixed gas composed of an inert gas (such as argon or helium) and hydrogen, and purification is accomplished by the removal of boron and carbon in the form of oxides.
U.S. Pat. No. 5,961,944 discloses production of silicon for solar cells by:
(a) melting silicon under vacuum to remove phosphorus by evaporation, and then solidifying the residue to form an ingot;
(b) removing the impurity concentrated portion of the ingot by cutting;
(c) re-melting the remaining portion, and removing boron and carbon by oxidizing under an oxidizing atmosphere, and then blowing an argon gas or a mixed gas of argon and hydrogen onto the melt;
(d) casting the deoxidized melt, followed by directional solidification to obtain a second ingot; and
(e) removing the impurity concentrated portion of the second ingot by cutting.
U.S. Pat. No. 4,097,584 discloses a method of purifying silicon to produce semiconductor grade silicon, comprising feeding, under pressure, a hydrogen-containing gas stream along with a small amount of water vapor through a body of molten technical-grade silicon.
A significant limitation currently on the use of solar cells has to do with the cost of purifying silicon to solar grade. In view of current energy demands and supply limitations, there is an enormous need for a more cost efficient way of purifying metallurgical grade silicon (or any other silicon having higher impurities than solar grade) to solar grade silicon.
The present invention is directed to a method of purifying silicon, comprising feeding a sparging gas into a liquid melt (also referred to as a liquid bath) containing molten silicon and at least one impurity.
In accordance with one aspect of the present invention, the sparging gas is fed into the liquid bath at a position between the bottom and the top of the bath.
In accordance with a second aspect of the present invention, the sparging gas is fed into a tube having a sparging gas exit end from which the sparging gas exits said tube into the liquid bath, the sparging gas exit end of said tube being located below the uppermost surface of the liquid bath.
In accordance with a third aspect of the present invention, the liquid bath is within a container and the sparging gas is fed into the liquid bath through a lid or sidewall in the container.
The sparging gas preferably comprises at least one gas selected from the group consisting of argon, oxygen, ammonia, nitrogen, chlorine, water vapor, hydrogen chloride, carbon dioxide, hydrogen, bromine and carbon monoxide.
In accordance with the present invention, the sparging gas is used to react with one or more impurity contained within the silicon. The products of such reaction may be removed by liquid-gas extraction (i.e., where the reaction product is volatile under the conditions in the silicon bath) or by liquid-liquid extraction (i.e., where the reaction product is less dense than the silicon, so that the reaction product rises into a surface slag which is formed on the surface of the liquid melt and is removed therefrom and/or where the reaction product is more dense than the silicon, so that the reaction product sinks into a bottom slag at the bottom of the melt and is later removed therefrom). Some reaction products are harmless to solar grade silicon, and can be left in the silicon bath and ultimately the purified silicon product.
In a preferred aspect of the present invention, by supplying the sparging gas to a location which is between the bottom and the top of the bath, there is created a more active reaction between the gas and the liquid (i.e. enhanced surface area of reaction). That is, the gas is rendered more reactive owing to the large surface area achieved by the bubbles formed in the liquid. One way to supply the sparging gas into the liquid bath at a location which is between the bottom and the top of the bath is to feed the sparging gas through a tube.
In another aspect of the present invention, the sparging gas preferably further comprises entrained solid silica.
The invention may be more fully understood with reference to the accompanying drawing figures and the following description of the embodiments shown in those drawings. The invention is not limited to the exemplary embodiments and should be recognized as contemplating all modifications within the skill of an ordinary artisan.
FIG. 1 is a sectional view of a first preferred embodiment of an apparatus for use in purifying silicon in accordance with the present invention.
FIG. 2 is a sectional view of a second preferred embodiment of an apparatus for use in purifying silicon in accordance with the present invention.
FIG. 3 is a sectional view of a third preferred embodiment of an apparatus for use in purifying silicon in accordance with the present invention.
FIG. 4 is a sectional view of an alternative embodiment of a tube for use in an apparatus for purifying silicon in accordance with the present invention.
FIG. 5 is a sectional view of a fourth preferred embodiment of an apparatus for use in purifying silicon in accordance with the present invention.
FIG. 6 is a sectional view of a fifth preferred embodiment of an apparatus for use in purifying silicon in accordance with the present invention.